Device to relieve thrust load in a rotor-bearing system using permanent magnets

ABSTRACT

The present invention provides a device and a method to enhance thrust load capacity in a rotor-bearing system. The load-enhancing device comprises a stator and a rotor arranged in such as way as to achieve a magnetic thrust load capacity enhancement by employing a number of permanent magnets, which produce an attracting force or an expulsing force between the rotor and the stator.

FIELD OF THE INVENTION

The present invention relates to rotor-bearing systems. Morespecifically, the present invention is concerned with a device and amethod to enhance thrust load capacity in a rotor-bearing system.

BACKGROUND OF THE INVENTION

In applications of high-speed rotor-bearing systems with significantthrust load, i.e. where there exists a significant load parallel to theaxis of rotation and tending to push the shaft in the axial direction,the design of thrust bearings that receive the longitudinal thrust orpressure of the shaft especially in so-called oil-free bearings systems,is generally a challenging task.

Magnetic bearings, which may be dimensioned to withstand the worstpossible operating conditions, as well as gas bearings, are often thefavorite candidates for such high-speed oil free applications.

However, a number of design difficulties has to be solved in order toobtain a high load capacity in the thrust bearing of a magnetic bearingsystem involves, such as the following for example:

-   -   a high load capacity means a large thrust area in the shaft, but        the thrust area is limited by a maximum outer diameter (“OD”)        due to a rotor material strength limit,    -   a high load capacity often requires large size coils and        magnetic flux path in a stator, resulting in a large axial        dimension of the stator, thus in turn requiring a longer rotor,        but the rotor length is limited by shaft mode frequencies; and    -   a high current is generally required in the coils, but it is        limited by the temperature rise in the winding. Moreover, high        current involves costly power electronics.

In gas bearing systems, such as hydrostatic or hydrodynamic systems, ahigh thrust load bearing requires a very large thrust area, whichoftentimes results unrealistic to build. In such systems, the mainproblems are due to a low viscosity of the gas, a low relative speedbetween rotor and bearings near the center of rotation, and a limitedpressure supply.

In rolling element thrust bearing systems and fluid bearing systems, ahigh load causes an increased system loss, consequently resulting in lowefficiency, and even overheating of the systems.

From the foregoing, there is obviously a need for a compact and highefficiency device and method to enhance thrust load capacity In arotor-bearing system.

OBJECTS OF THE INVENTION

An object of the present invention is therefore to provide an improveddevice and a method to enhance thrust load capacity in a rotor-bearingsystem.

SUMMARY OF THE INVENTION

More specifically, in accordance with the present invention, there isprovided a thrust load enhancement device for a rotor-bearing systemcomprising:

a stator mounted on a rotation axis of the rotor-bearing system;

a rotor separated from the stator by a first air gap on the rotationaxis; and

at least one permanent magnet separated from the rotor by a second airgap,

wherein the at least one permanent magnet, the stator and the rotor forma magnetic circuit characterized by a flux path so that a flux in thefirst and second air gaps generates a compensation force between therotor and the stator that opposes an external force F_(ext).

There is further provided a method for thrust load enhancement for arotor-bearing system comprising the steps of:

providing a stator on a rotation axis of the rotor-bearing system;

providing a rotor separated on the rotation axis from the stator by afirst air gap; and

providing at least one permanent magnet separated from the rotor by asecond air gap,

whereby the at least one permanent magnet, the stator and the rotor forma magnetic circuit characterized by a flux path so that a flux in thefirst and second air gaps generates a compensation force between therotor and the stator that opposes an external force F_(ext).

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of embodiments thereof, given by way of example only withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the appended drawings:

FIG. 1 is a cross-section of a load enhancement device according to afirst embodiment of a first aspect of the present invention;

FIG. 2 is a cross-section of a load enhancement device according to asecond embodiment of the first aspect of the present invention;

FIG. 3 is a cross-section of a load enhancement device according to athird embodiment of the first aspect of the present invention;

FIG. 4 is a cross-section of a load enhancement device according to afirst embodiment of a second aspect of the present invention; and

FIG. 5 is a cross-section of a load enhancement device according to afurther embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Generally stated, the present invention provides a device and a methodto enhance thrust load capacity in a rotor-bearing system.

More precisely, the load enhancement device according to a first aspectof the present invention comprises a stator and a rotor in such a way asto achieve a magnetic thrust load capacity enhancement by employing anumber of permanent magnets, which produce an attracting force betweenthe rotor and the stator that opposes a force applied from the outsideand referred to hereinafter as F_(ext). Such an outside force F_(ext)can be caused by pressure or gravity in a vertical shaft configurationwherein the center of gravity of the configuration is low, for example.

Turning now to FIGS. 1 to 3 of the appended drawings the loadenhancement device according to various embodiments of the first aspectof the present invention will now be described.

The load enhancement device comprises a stator 14 and a rotor 12arranged so as to obtain attracting compensation forces between thestator 14 and the rotor 12.

Turning to FIG. 1, as a first embodiment, the load enhancement devicecomprises the rotor 12, a stator pole piece 14, a permanent magnet 16,and a spacer 18.

The permanent magnet 16 is fixed to the stator pole piece 14 In such away that the permanent magnet 16, the stator pole piece 14 and the rotor12 form a magnetic circuit wherein the stator pole piece 14 and therotor 12 are separated by a gap, as well as the rotor 12 and thepermanent magnet 16.

The resulting magnetic circuit is characterized by a flux path, shown inFIG. 1 as dash lines. The flux in the air gaps between the stator polepiece 14 and the rotor 12, and the rotor 12 and the permanent magnet 16respectively, generates an attracting force that is able to compensatethe external force F_(ext).

By optimizing the geometry of the various soft magnetic pole faces,magnet and air gaps, a minimum volume of magnet can be used, underconstrains of sizes and air gaps. When the disposition of the permanentmagnet 16 in relation to the soft magnetic poles are thus fixed, thespacer 18 allows to adjust the air gaps in order to vary thecompensation, since it is a well known physical rule that the magneticforce increases when the air gap decreases. The adjustment offersflexibility to handle modeling accuracy, manufacturing and materialtolerances, and process variation.

In a second embodiment shown in FIG. 2, the load enhancement device isbasically similar to that shown in FIG. 1. The only difference is thatthe attracting force between the rotor 12 and the stator 14 is createdby a magnet 16 mounted in the rotor 12.

The rotor 12 is made from a soft magnetic material such as carbon steel.The stator pole piece 14 is also made from a soft magnetic material,such as mild steel, for example.

As shown in a third embodiment in FIG. 3, the attracting force betweenthe rotor 12 and the stator 14 is created by a first magnet 16 a mountedin the rotor 12, and a second magnet 16 b mounted in the stator 14, eachmagnet 16 a, 16 b respectively having poles of different polarity facingeach other. Alternatively, in cases where the rotor 12 and the stator 14are made from soft magnetic materials, the attracting force between therotor 12 and the stator 14 can be created by arranging pole facesbetween the rotor 12 and the stator 14.

According to a second aspect of the present invention, a loadenhancement device is provided that comprises a stator and a rotor insuch a way as to achieve a magnetic thrust load capacity enhancement byemploying a number of permanent magnets to produce an expulsion forcebetween the rotor and the stator.

As shown in FIG. 4, an expulsion force can be created by a first magnet16 a mounted In the rotor 12, and a second magnet 16 b mounted in thestator 14, the magnets 16 a and 16 b being arranged with poles of asimilar polarity facing each other, for example with a pole N of themagnet 16 a facing a pole N of the magnet 16 b.

From the foregoing, it appears that either an attracting force or anexpulsion force can be generated by using two magnets, by varying thearrangement of the polarities of the various magnets, depending on thedirection of external forces to be dealt with.

In both cases when the load enhancement device of the present inventioncomprises a magnet fixed in the rotor and a magnet fixed in the stator(see FIG. 3 and FIG. 4), the rotor 12 and stator 14 may be made ofnon-magnetic materials. If soft magnetic materials are used in rotor 12and stator 14, the geometry of the magnets and the arrangement of thepole faces and air gaps may be optimized to use a minimum volume ofmagnets, thereby achieving compactness and cost savings. Indeed, if softmagnetic materials are used for the stator 14 and the rotor 12, theforce generated in the air gap between them also contributes to thecompensation force. Therefore, less magnet material is required.However, such an alternative may prove rather expensive, since softmagnetic materials may be quite expansive.

Again, the spacer 18 allows to vary the air gaps, and hence to adjustthe compensation force.

As can be seen in FIG. 5, if an automatic or in-situ adjustment of thecompensation force is required, a piezoelectric actuator 20 may be used(instead of a spacer) to adjust the air gaps of the load enhancementdevice of the present invention, which in turns alters the compensationforce.

People in the art will appreciate that the configuration of FIG. 5 mayalso be used to compensate an external dynamic force if a dynamiccompensation signal is applied to the piezoelectric actuator 20.

Moreover, force measurement devices 22, such as stain gauges orpiezoelectric elements, may be applied to the load enhancement device asshown in FIG. 5 to measure the compensation force. Such an option mayprove useful In monitoring applications. For applications of activemagnetic bearing systems, the force (dynamic and static) delivered bythe active bearing is inherently available. Using a force measurementdevice as illustrated in FIG. 5, the force compensated by the loadenhancement device of the present invention is therefore measured. As aresult, the total external force applied to a shaft can be obtained.

According to a third aspect of the present invention, there is provideda method of obtaining different level of capacity enhancement byadjusting the magnetic air gap between the stator and rotor. Indeed, theload enhancement device described hereinabove allows generating a forcebetween the stator and the rotor that compensates an external forceF_(ext).

Such an adjustment can be achieved by using a spacer (see FIG. 1), orautomatically achieved by means of an actuator, for example apiezoelectric element, mounted in the stator as is shown in FIG. 5.

The force delivered by the load enhancement device and method of thepresent invention may be measured by either a stain-gauge or by apiezoelectric element (FIG. 5), for example.

As a way of example, the method according to an embodiment of the thirdaspect of the present invention comprises using soft magnetic materialsto build the rotor and the stator, thereby optimizing usage of magnetsgenerating a compensation force (see FIGS. 1 to 4 for example); using aspacer for adjustment of the compensation force; using a piezoelectricactuator to automatically adjust the compensation force (be it a staticand/or dynamic force); using a stain gauge or a piezoelectric element tomeasure the compensation force; placing the load enhancement device atan end of a shaft, thereby not requiring modification of the shaftlength.

As people in the art will understand from the foregoing, theconfigurations described hereinabove may be varied according to specificapplications. For example, when it is important to minimize the shaftlength, the configuration illustrated in FIG. 1 may be advantageous.

People in the art will appreciate that the method of the presentinvention allows a magnetic thrust load capacity enhancement, whileavoiding the use of either solid or fluid contacts, in other words, bynon-contact means.

Interesting applications of the present invention are in systems wherethe thrust load is unidirectional either from an external working loador a rotor weight in a vertical configuration.

People in the art will appreciate that the load enhancement device andmethod of the present invention may be used in a magnetic bearingsystem, a hydrostatic bearing system, a hydrodynamic bearing system, ora rolling element bearing system for example. As a specific example, thepresent invention may be applied to compensate a unidirectional externalstatic load such as a working load, e.g. static pressure, or a shaftweight in a vertical configuration.

Therefore a compact, low cost thrust force handling (thrust bearing andthrust force enhancement device) may be achieved according to theteachings of the present invention.

Moreover, dynamic load compensation is also possible if the dynamic loadis measured and an actuator is implemented as shown in FIG. 5.

It should be noted that since the load enhancement device of the presentinvention may be positioned at one end of a shaft (see in FIGS. 1 to 5),the shaft length need not be modified to accommodate the loadenhancement device.

People in the art will appreciate that the present invention does notintroduce any friction loss due to a direct contact using rollingelement thrust bearings for example, or fluid coupling such as fluidtype thrust bearings. Moreover, since significantly larger gaps betweenthe rotor and the stator may be used in comparison to the case of thrustbearings systems, the present invention allows minimizing windinglosses.

Although the present invention has been described hereinabove by way ofembodiments thereof, it may be modified, without departing from thenature and teachings thereof.

1. A thrust load enhancement device for a rotor-bearing system, comprising: a stator mounted on a rotation axis of the rotor-bearing system; a rotor separated from said stator by a first air gap on the rotation axis; and at least one permanent magnet separated from said rotor by a second air gap; wherein said at least one permanent magnet, said stator and said rotor form a magnetic circuit characterized by a flux path, a flux in said first and second air gaps generating a compensation force between said rotor and said stator that opposes an external force F_(ext).
 2. The thrust load enhancement device according to claim 1, wherein the external force F_(ext) is caused by an action selected from the group consisting of pressure and gravity in a vertical shaft configuration wherein a center of gravity is low.
 3. The thrust load enhancement device according to claim 1, wherein said at least one permanent magnet is fixed to said stator.
 4. The thrust load enhancement device according to claim 1, wherein said at least one permanent magnet is fixed to said rotor.
 5. The thrust load enhancement device according to claim 1, wherein a first one of said at least one permanent magnet is fixed to said stator and a second one of said at least one permanent magnet is fixed to said rotor.
 6. The thrust load enhancement device according to claim 5, wherein said first one of said at least one permanent magnet and said second one of said at least one permanent magnet respectively have poles of different polarity facing each other to create an attractive compensation force between said rotor and said stator.
 7. The thrust load enhancement device according to claim 5, wherein said first one of said at least one permanent magnet and said second one of said at least one permanent magnet respectively have poles of a similar polarity facing each other to create an expulsion compensation force between said rotor and said stator.
 8. The thrust load enhancement device according to claim 1, further comprising a spacer to adjust said first and second air gaps.
 9. The thrust load enhancement device according to claim 1, further comprising a piezoelectric actuator mounted in said stator.
 10. The thrust load enhancement device according to claim 1, wherein said rotor and said stator are made in a material selected from the group consisting of a soft magnetic material and a non-magnetic material.
 11. The thrust load enhancement device according to claim 1, wherein said rotor is made of carbon steel and said stator is made of mild steel.
 12. The thrust load enhancement device according to claim 1, wherein the external force is selected in the group consisting of a static force and a dynamic force.
 13. The thrust load enhancement device according to claim 1, further comprising force measurement devices to measure the compensation force.
 14. The thrust load enhancement device according to 13, wherein said force measurement devices are selected from the group consisting of stain gauges and piezoelectric elements.
 15. The thrust load enhancement device according to claim 1, wherein said load enhancement device is located at one end of a shaft of the rotor-bearing system.
 16. The thrust load enhancement device according to claim 1, wherein the thrust load is unidirectional from an external working load.
 17. The thrust load enhancement device according to claim 1, wherein the thrust load is unidirectional from a rotor weight in a vertical configuration.
 18. The thrust load enhancement device according to claim 1, wherein the external force is an unidirectional external static load selected from the group consisting of a working load and a shaft weight in a vertical configuration.
 19. The thrust load enhancement device according to claim 1, wherein the rotor-bearing system is selected from the group consisting of a magnetic bearing system, a hydrostatic bearing system, a hydrodynamic bearing system, and a rolling element bearing system.
 20. A method for thrust load enhancement for a rotor-bearing system comprising the steps of: providing a stator on a rotation axis of the rotor-bearing system; providing a rotor separated on the rotation axis from the stator by a first air gap; and providing at least one permanent magnet separated from the rotor by a second air gap, whereby the at least one permanent magnet, the stator and the rotor form a magnetic circuit characterized by a flux path so that a flux in the first and second air gaps generates a compensation force between the rotor and the stator that opposes an external force F_(ext).
 21. The method for thrust load enhancement according to claim 20, wherein said steps of providing a stator and said step of providing a rotor comprise providing a rotor and a stator made in a material selected from the group consisting of a soft magnetic material and a non-magnetic material.
 22. The method for thrust load enhancement according to claim 20, wherein said step of providing a stator comprises providing a stator made of mild steel and said step of providing a rotor comprises providing a rotor made of carbon steel.
 23. The method for thrust load enhancement according to claim 20, wherein said step of providing at least one permanent magnet comprises mounting at least one permanent magnet on the stator.
 24. The method for thrust load enhancement according to claim 20, wherein said step of providing at least one permanent magnet comprises mounting at least one permanent magnet on the rotor.
 25. The method for thrust load enhancement according to claim 20, wherein said step of providing at least one permanent magnet comprises fixing a first one of the at least one permanent magnet to the stator and a second one of the at least one permanent magnet to the rotor.
 26. The method for thrust load enhancement according to claim 25, wherein said steps of fixing a first one of the at least one permanent magnet to the stator and a second one of the at least one permanent magnet to the rotor comprise arranging respective poles of different polarity thereof facing each other to create an attractive compensation force between the rotor and the stator.
 27. The method for thrust load enhancement according to claim 25, wherein said steps of fixing a first one of the at least one permanent magnet to the stator and a second one of the at least one permanent magnet to the rotor comprises arranging respective poles of similar polarity facing each other to create an expulsion compensation force between the rotor and the stator.
 28. The method for thrust load enhancement according to claim 20, further comprising a step of providing a spacer to adjust said first and said second air gaps.
 29. The method for thrust load enhancement according to claim 20, further comprising the step of mounting a piezoelectric actuator in the stator.
 30. The method for thrust load enhancement according to claim 20, wherein the external force is selected from the group consisting of a static force and a dynamic force.
 31. The method for thrust load enhancement according to claim 20, further comprising the step of providing force measurement devices to measure the compensation force.
 32. The method for thrust load enhancement according to 31, wherein said step of providing force measurement devices comprises selecting devices from the group consisting of stain gauges and piezoelectric elements.
 33. The method for thrust load enhancement according to claim 20, wherein the rotor-bearing system is selected from the group consisiting of a magnetic bearing system, a hydrostatic bearing system, a hydrodynamic bearing system, and a rolling element bearing system. 